It is known in connection with subterranean galleries or tunnels to coat the excavated wall with a layer of concrete, to protect ducts and the like with concrete coating and in general to provide a surface to be concreted with a coating of the flowable concrete mixture which ultimately hardens to form a concrete layer on this surface.
The concrete generally consists of hydraulic cement, an aggregate, a filler which is usually a more finely divided mineral matter than the aggregate which can be gravel or crushed rock, additives of various kinds, e.g. metal fibers or synthetic resins serving to reinforce or reduce the density of the concrete mass, or materials serving to accelerate setting or otherwise control the setting time, and water in a permeable and sprayable concrete mixture.
In current practice, the concrete mixture is pumped to a spray head which has a nozzle from which a jet of the concrete can emerge and which is equipped with means connecting that nozzle to a source of compressed air, also referred to as driving air, which assists in propelling the composition in the form of a spray from the nozzle and permits the particles of that spray to collect on the surface to be coated or concreted.
The concrete pump supplies the flowable concrete mixture at a predetermined volume flow rate to the nozzle and the driving air source supplies the driving air at a pressure of generally several bars to the nozzle.
When reference is made herein to a filler, we intend to so identify finely divided fillers commonly used in spray concretes, especially fly ash. As noted, the additives when they are used, can be setting accumulators or setting retarders. They can include synthetic resins or even synthetic resin components which can polymerize in situ to serve to modify the structure of the applied concrete or to promote some favorable characteristic or reduce some unfavorable characteristic of the hardenable composition.
A reference to the driving pressure herein will be understood to mean the static pressure of the driving air supplied by the compressed air source and delivered to the inlet for the driving air of the spray head and thus the static pressure at the entry to the latter.
While such systems have been used in the past to considerable advantage, both the methods and the apparatuses which have been employed leave much to be desired. For example, the setting time is relatively high. To render the composition both permeable and sprayable, it is frequently necessary to include in the composition excess water, i.e. water which does not participate in the hydration reaction which the concrete composition undergoes for setting. The excess water has been found to delay the setting time and this, in turn, has an adverse influence on the thickness in which the composition can be applied in a single spray pass and on the overall thickness of the concrete layer which be applied to surface.
To reduce the setting time, it is known to admix water glass. to the concrete composition and this is generally done upon make up of the original composition.
Water glass, however, has the drawback that it is an additive which promotes a chemical hardening process which must be superimposed on the usual hydration reaction which the composition is to undergo. It tends to raise the pH value of the fresh concrete significantly above its normal pH of 12.6 and to promote undesirable alkali silicate reactions which tend to reduce the strength of the setting concrete as a function of time, i.e. limits the fast-hardening strength of the concrete.
From United Kingdom patent specification No. 2,020,722 it is known to add silica dust to a spray concrete composition.
Silica dust is defined as a dust-fine particulate of amorphous or colloidal silica which is usually fabricated as precipitation silicic acid or by high temperature hydrolysis and can be distinguished because of its amorphous character from crystalline sands otherwise known as quartz sands or silica sands.
In this system, the cement composition in the form of a flowable mixture of the hydraulic cement, possibly fine-grained fillers and water, can be latter mixed with the dry aggregate. The silica dust is introduced into the flowable mixture. The early contact of the silica dust with water and the long time between the introduction of the silica dust into the mixture and emplacement of the concrete here practically ensures that no significant effect of the adsorption heat will contribute to promotion of a setting reaction. Indeed the reference does not even mention internal surface area or specific surfaces of the silica dust.
As a consequence, the silica powder does not materially affect the setting time although it does contribute to an increased strength of the setting concrete. Presumably the silica powder reacts with calcium hydroxide of the composition to form calcium silicate hydrates which favorably affect the crystal structure of the concrete when it is set.